CPP: Add '/' case.

cpp/ql/src/semmle/code/cpp/File.qll

@@ -313,6 +313,8 @@ class File extends Container, @file {
       getAbsolutePath().charAt(1) = ":"
       	// this is not ideal, it detects compilation on a Windows file system
       	// as a heuristic approximation to compilation with a Microsoft compiler.
+    ) or (
+      getAbsolutePath().charAt(1) = "/"
     )
   }